It's 10.40 on a Sunday night in Gateshead as a woman stands alone at a station waiting for a train. A young man steps out of the shadows, grabs her and sexually assaults her, before escaping into the night. The attacker is caught on CCTV and police soon have excellent pictures.

So what do they do? Do they release the story to the media so they can warn other women to be on their guard – and help track down the suspect ? No. They wait nearly two weeks before they tell journalists. So what reason did they have for not issuing a warning and using the media to solve a crime? "There were a number of lines of inquiry to follow up in connection with this incident," says a police spokeswoman. And that's it … that's all we are allowed to know.

The holding back of the details of one crime in the north-east may seem to be no great deal. However, in fact research, carried out over several years, has revealed forces are not disclosing 99% of crimes to the media. When they do release information, it's often only as a last resort when all other inquiries have been exhausted.

Headlines about phone hacking and detectives allegedly leaking confidential information to journalists have conjured up images of a cosy and corrupt relationship between the police and the media. But my research shows that all too often journalists are being kept in the dark about crimes that the public should be told about. This phenomenon is not restricted to my native north-east, either.

Investigations by the Oxford Mail, the Hull Daily Mail and the York Press have shown the same proportion of crimes go unreleased by their local forces. Adam Lovell, crime reporter at the Hull Daily Mail, says: "Last year, we had a case involving the attempted abduction of a four-year-old girl. They had an e-fit of the suspect but they took 10 days to even tell us it had happened."

Humberside police claimed they had held back because there were "other lines of inquiry" and they did not want to risk "scaremongering". But Lovell says: "This wasn't an unusual case. There are hundreds of serious crimes they don't tell us about – or they come to us weeks later as a last resort after all inquiries have failed. We find out about crimes which have happened from the victims, rarely because the police tell us."

Simon O'Neill, editor of the Oxford Mail and a former crime reporter, has a similar problem with Thames Valley police. He says: "There is a control freak culture with regard to issuing information about crime to the press and public. They often don't even bother to tell us about major crimes that have happened. We often only find out after a reader calls in or demands to know why we haven't covered it. We had a robbery at a pub. It happened in March but the information was only released, with an e-fit, in June."

But does it matter if the police hold back information from journalists? There are two key reasons why they should keep the media informed. Firstly, the public have a right to know what is happening in their communities. Secondly, the police usually stand a better chance of catching the culprits if they use the media to appeal for witnesses.

Over a typical weekend in March, my local force, Northumbria, released: a minor road accident; a robbery at a shop; a stolen car; a stolen dog; and an appeal regarding an assault from a week earlier. However, a request under the Freedom of Information Act revealed there were 674 crimes, including 55 cases of grievous bodily harm, 20 other assaults, one armed robbery and three other robberies, five rapes, 12 other sexual assaults and 69 burglaries.

While not denying that less than 1% of crimes are publicised, Northumbria denies it is hiding crimes from the public, pointing out all crime figures are eventually released on their crime-mapping website.

One suggested explanation for the policy is that senior officers are under pressure from "public confidence" targets brought in by the last government. Northumbria, for example, recently boasted the highest public confidence figures in the country, with 66% of those questioned claiming the force was tackling their concerns. But how much of this success is down to what forces tell – and don't tell – the media?

Andy Trotter, the chief constable of British Transport Police, is the Association of Chief Police Officers' spokesman on media affairs. He denies claims that forces have grown more secretive: "I think the police have improved hugely from when I joined 40 years ago. When I started, the press were treated with great suspicion. I think the accessibility of the police is far greater than it's ever been, [and] we are more open with the media than ever before in dealing with major crimes or incidents."

What is true is that since the late 1990s, there has been a massive expansion in public relations across the private and public sector and police forces are no exception. Corporate communications and media services spending rose by around 21% between 2005 and 2009. Last year, forces across England and Wales alone spent more than £33m.

Media services managers say the increase has been due to increased demands from journalists, including 24-hour TV news channels and websites. Maureen Berne, head of corporate communications at Northumbria, recalls an "unprecedented" demand for information during the search for Raoul Moat, the lone gunman who eventually killed himself after several days on the run. "From the initial shootings through to Moat being discovered in Rothbury, the media office dealt with over 1,000 inquiries."

Nevertheless, while some high-profile cases require considerable resources, the prevailing impression is of a culture of concealment. Some journalists have proposed guidelines to tackle that by measuring how much information forces put out. In the past, Bob Satchwell, who runs the Society of Editors, has spoken to Her Majesty's Inspectorate of Constabulary and suggested setting targets for how much information forces release. Satchwell says: "We should be able to measure how good the relationships are with the public and the media and that should be looked at as part of an annual inspection."

However, such hopes of improving relationships have to be set against the prospect of forces having to make cuts. Recent research shows the rise in spending on media services departments is over. Using the Freedom of Information Act, it emerges that of the 42 forces surveyed, nearly three out of four revealed they had cut the amount they spend on media services departments this year.

The force showing the most dramatic cut is South Yorkshire, which is down from £789,000 to £493,000 (38%). Another cutting back heavily is Thames Valley, which had a budget of £1,643,000 last year but planned to spend £1,218,020 this year (a 26% cut).

The way the Met handles the media has come under close scrutiny since it was revealed it employed Neil Wallis (pictured), the former deputy editor of the News of the World and one of those arrested over the phone hacking scandal. Indeed the Met's corporate communications budget hit £6.7m last year, although it has been pegged back to £6.5m in the latest cuts. But such cuts are relatively small when compared to those suffered by many UK newsrooms.

Widespread redundancies have taken a toll in both the quantity and quality of journalists available to check and challenge what the police are or are not telling them. O'Neill says: "There is a danger that newspapers are just going to shovel the crap the police want them to shovel. When you balance the fact that resources are decreasing, it is the easiest option. It is exhausting for my team trying to extricate basic information that the public have an absolute right to know."

There is a danger that the phone-hacking scandal and the allegations of police corruption may mean we take our eye off the ball. We should rightly be concerned about police passing on sensitive and personal information. But, when it comes to crime on our streets, there is perhaps an even greater risk when the police don't tell us what we have a right to know.